Radio control circuit



0ct. 5, 1954 F. L. HILTON 2,691,094

RADIO CONTROL CIRCUIT Filed Sept. 26. 1951 2 Sheets-Sheet 1 Filed sept. 26. 1951 2 Sheets-Sheet 2 NCQ Busi Hl/fon INVENTOR Frederic Afb' Patented Oct. 5, 1954 RADIO CONTROL CIRCUIT Frederick L. Hilton, Bensonvillc, Ill., assignor to Motorola, Inc., Chicago, Ill., a corporation of Illinois Application September 26, 1951, Serial No. 248,410

(Cl. Z50- 13) 9 Claims.

The present invention relates generally to automatic control circuits for radio communication systems and more particularly to antenna switching and power sequence control circuits for a local or remotely controlled radio communication system.

In the operation of remotely controlled radio frequency transmitters of the type having separately energized low power exciter and high power amplifier stages together with their associated direct current plate power supply circuits, it has been found desirable to sequence the energization of the various stages in order to prevent damage to the circuit components. For example, it is desirable to secure full energization and operation of the exciter stage before the power amplifier stage is energized. Similarly, it is desirable, when turning off the transmitter, to deenergize the power amplifier stage before deenergizing the exciter stage thus preventing such damage to l the power amplier tubes as might result from their energization with no excitation of their control grids. When such remotely controlled transmitter systems employ tubes cooled by forced air, it is also desirable to provide means for automatically deenergizing the entire transmitter in the event of failure of the air blower system.

Remotely controlled radio frequency transmitters of the type referred to above are frequently used as vital links in important communication services such as police and fire dispatching systems and it is therefore desirable to provide for continued operation of the transmitter at low power in the event that high power operation is inadvisable or impossible due to overload conditions in the nal high power amplifier. Such operation may be obtained when using a transmitter circuit combination of low power exciter and final power amplifier, by switching the transmitting antenna from the final power amplifier to the output of the exciter and at the same time deenergizing the power amplifier.

It is a principal object of the present invention to provide a radio frequency communication system that is automatically controlled to continue transmitter operation at low power in the event that high power operation is inadvisable or impossible due to overload conditions.

Another object of the invention is to provide a radio frequency communication system that is automatically controlled to continue transmitter operation at low power in the event of overload and which will automatically be restored to full transmitting power after the conclusion of the low power transmitter operation and the beginning of a successive period of transmission.

Yet another object of the present invention is to provide an automatically controlled radio frequency communication system which is automatically operable with the same antenna for the transmitter under low or high power operation and for receiving.

A further object of the present invention is to provide .an automatic sequencing system for controlling the energization and deenergization of the various transmitter stages in desired sequences to minimize damage to the various stages due to improper loading or other conditions.

A still further object of the present invention is to provide a simple and automatic control circuit for a radio frequency communication system which permits selection of either remote or local control, and which facilitates tune-up procedures under local control.

A principal feature of the present invention is the provision of an automatic control system for a radio transmitter having separately energized low and high power amplifier stages, which control system will be automatically responsive to overload conditions of the high power amplifier to deenergize such amplifier and automatically switch the antenna from the output circuit of the high power amplifier to the low power output circuit.

A further feature of the invention is the provision of a control system for a transmitter as described above, which automatically reenergizes and reconnects the power amplifier to the antenna after previous disconnection due to overload conditions, and which is operative upon release of the push-to-talk switch and a subsequent reclosing of the push-to-talk switch. Provisions may also be included to automatically switch the transmitting antenna from the transmitter or eX- citer to a receiver whenever the push-to-talk switch is released.

Another feature of the present invention is'the provision of an improved power control system for `a remotely controlled radio frequency transmitter including separately energized exciter and power amplifier stages, the control system having means to automatically and sequentially energize first the plate circuits of the exciter and thereafter the plate circuits of the power amplier and to sequentially deenergize the exciter and power .amplifier plate circuits in reverse sequence whenever the remote control push-to-talk switch is manipulated to energize or deenergize the transmitter as the case may be.

The communication system of the present invention also provides a novel power control circuit for a remotely controlled forced-air-cooled transmitter which automatically deenergizes the power amplifier of the transmitter in the event of failure of the air blower in order to subsequently reenergize the power ampliiier from either a local or remote position.

In a preferred embodiment of the invention the transmitter control system is provided with a manual selector switch to be operated to enable local control of the energization of the power amplier plate circuits by a local push-to-talk microphone switch and further includes automatic means for switching the antenna to the output of the power amplier when the power amplifier is energized together with means assuring the connection of the power amplifier output to the antenna at all times that the power ampliner is energized except during times when overload conditions exist in the power amplifier.

Further objects, features, and advantages of the present invention will be apparent from a consideration of the following specication and drawings in which:

Fig. 1 is a schematic diagram of the transmitter control system and relays connected for controlling the exciter, power amplifier and power amplifier power supply; and

Fig. 2 is a schematic diagram showing a modified form of the invention in which is shown a radio communication system having a control circuit providing for automatic switching of the antenna from transmitter to receiver.

The invention in its preferred embodiment comprises a radio communication system having an automatic control system including a plurality or" switches and relays for energizing and deenergizing the various stages of the transmitter in desired sequences. The transmitter proper consists of separately energized low and high power stages and an antenna. Switch means responsive to an overload condition in the high power stage is provided to automatically control the energization of the high and low power stages and to switch the antenna from the high to the low power stage to enable continued operation of the transmitter at low power. The control system including the overload responsive switch may be operated by the push-to-talk switch of either a local or a remote microphone. The system is effective to reenergize the high power stage and again switch the antenna thereto after the completion of a transmitting period and the beginning of a successive period of transmission. If the overload condition still exists, the transmitter will again be switched to low power operation. The control system is also provided with a manual selector switch having three positions of operation to enable the control system to function by remote control, by local control, and to energize only the exciter or low power circuit for tune-up purposes. The transmitter is cooled by forced air and the control system includes an air switch that will deenergize the power amplier of the transmitter in the event of air failure.

In another embodiment of the invention the communication system includes a receiver which is operative during periods when the transmitter is not operative. The control system in this embodiment further provides antenna switching means to automatically connect the antenna to the receiver whenever the transmitter push-totalk control switch is released.

Referring to Fig. l of the drawings, an exciter I0, final power amplifier II, and power ampliiier power supply i2 which may be conventional units are shown in block form. A transmitting antenna I3 is shown to be connected to either the output of the power amplier by line i4, or the output of the exciter by line I5, through means of the antenna switch contacts I6 and Il controlled by relays I2 and I8, respectively. Antenna switch contacts I5 and Il are of the interlocked type that are operated simultaneously to close one and open the other upon the energization of either relay coil I8 or relay coil IS, respectively.

A main power switch 2l), when closed, energizes the volt relay coil 2| by connecting the coil from the line 25 to the plus 110 volt terminal. rlhe energization of relay coil 2I closes switch contacts 23 to connect the air blower motor 24 with the 110 volt lines 25 and 2S thus starting the operation of the blower motor. As soon as the blower motor 2li. and fan 2l' reach their normal operating speed the air flow switch contacts 2S are closed to connect the relay coil 30 across the 110 volt lines 25 and 3i, thus closing the contacts 32, to be mentioned later.

Conventional forms of cabinet interlock switches 36 and 3l are shown in their closed position. It should be understood that when the cabinet doors of the power amplifier II are opened, the interlock switches 3B and 31 will also be opened to prevent the application of plate voltage to the power amplifier as will be presently described.

The power control circuit of the exciter I0 and its operation for purposes of tuning the exciter will now be described. Assuming the main power switch 2t, and relay switches 23 and 32 to be closed, the blower motor 24 energized so that air operated switch 23 is closed, and the interlock switches 36 and 3'! closed as in the normal transmitter warm-up operation, the transmitter control switch it is now manually closed to energize the six volt relay coil 4I from the line 22 through interlock switches 35, 3l, switch contacts 32 and lines l2 and 633 to ground. It will be noted that the antenna switch relay coil I9 has been energized from the 110 volt line 25 through lines LlSa, l and 45, the normally closed switch contacts 46, line 4l and normally closed switch contacts 48 and 49 to 110 volt line 3 I, but the present energization of relay coil 4I opens switch contacts 49 to deenergize the antenna switch coil I9 and thus condition switch I 'i to be opened when relay I8 is subsequentlyenergized to close switch I6 and transfer antenna I3 from the line |15 to the line I4. Now, upon closing the local microphone or push-to-talk switch 50, the six volt relay coils 5I and 52 will be energized from the six volt line 53 to ground through line 54 and the normally closed door interlock switches 55. Switch 56 is also an interlock in the exciter cabinet. The energization of relay 5I closes contacts 5l' and connects power from the power source 58 through line 59 to the exciter I thus energizing the exciter.

It will be noted that a multiple circuit phone type manual selector switch 6D is shown, having three positions, a normal mid-position for remote control operation of the transmitter, an exciter tune-up position to the right on the drawing, and a local transmitter control position to the left on the drawing. With the manual selector switch G0 moved to the right in the exciter tune-up position, the six volt relay coil 6I is energized from the six volt line 53 to ground through line 62, now closed switch contacts 63, lines 54 and 65 and the now closed switch contacts S6. The energization of relay coil 6I closes switch contacts 6l and opens switch contacts G'IA to connect the output of the exciter I9 through lines 68 and 69 to the input of the power amplifier thus properly loading the exciter output circuits for purposes of tune-up. After tuning, the exciter I may be deenergized by opening the local push-to-talk switch 50 and deenergizing relay 5I to open contacts 51 and break the power supply circuit to the exciter.

Now assuming the exciter to be properly tuned, the details and operation of the transmitter control system under local control will be described. The manual selector switch 60 is moved to the left or local control position and it will be assumed that all switch contacts 28, 23, 28, 32, 36, 31 and 4D are closed as previously described for the transmitter warm-up operation, the relay coil 6I is now connected in parallel with relay coil 1I through lines 64, 65, now closed switch contacts 'I2, 13, lines 14, 15, normally closed switch contacts 18 and lines 42 and 43. When the local push-to-talk microphone switch 50 is closed, relays 5| and 52 are again energized as previously described and switch contacts 11 are closed to connect lines 'I8 and 65 to ground. In such manner both the six volt relay coils 6I and Il are energized from the six volt line 22 through closed switches 48, 31, 36, 32, and 16 together with lines 42, 49, 14, 15, 64, 85 and closed switch 'I3 through line 'I8 and closed switch contacts 11 to ground. The energization of relay 6| connects the output of the exciter to the input of the power amplifier as previously described. The energization of relay 1| opens switch contacts 48 to assure deenergization of the antenna switch relay I9, and simultaneously closes switch contacts 88 to connect the 110 volt antenna switch relay coil I8 to 110 voltl line 25 through line 43a, and to the '110 volt line 3| through line 8|, and normally closed switch contacts 82 and line 83. Relay coil I8 closes contacts I6 to connect the antenna I3 to the output of the power amplifier I I. The energization of antenna switch relay coil I8 also closes switch contacts 84 to energize the 110 volt relay coil 85 through lines 43a and 8| and close the switch contacts 85 to apply power in line 81 from the power source 38 to the power amplifier plate power supply I2 and thus energize the power amplifier I I. It will also be noted that the normally closed switch contacts 4B are opened by the energization of relay coil 85 to provide further assurance that antenna switch relay coil I9 will be deenergized and the output of the transmitter power amplifier II will be connected to the antenna. Switch contacts 48 also perform a function to be later described in connection with the automatic antenna switchover under overload conditions.

As described above, when the manual selector switch 58 is in the left or local control position and the main power and transmitter control switches 20 and 48 are closed, and assuming the blower motor is operating properly to close the air switch contacts 28, the transmitter may be energized and operated under local control by closing the local push-to-talk switch 50. In the event of overload in the power amplifier, the overload relay diagrammatically shown at 99 will become operative. The overload relay 98 may be responsive to the current drawn by the power amplifier I I from the power supply I2 and thereby operate when the amplifier is overloaded. The relay 98 is connected with the relay 9| which thus far in the normal operation of the transmitter has remained deenergized. The interconnection between the relays and 9| may be either mechanical or electrical and are represented by the dotted line in Fig. 1. The six volt relay 9| serves to provide a holding circuit for the overload relay 98 so that the operation of the overload relay 90 opens normally closed switch contacts 16 and closes switch contacts 92 to energize the six volt holding coil 9| through lines 42 and 15, closed switch contacts 13, line 18 and closed switch contacts 11 to ground. (Manual selector switch 60 in the local control position.) With the overload relay holding coil 9| energized, under conditions of overload in the power amplier, and the normally closed contacts 82 thereby is opened, relay 85 is deenergized and opens switch contacts 86 to disconnect the power to the power amplifier power supply I2. At the same time, the transmitter antenna switch relay I8 is deenergized to condition the antenna switch for automatic switchover to the output of the exciter. The previously described deenergization of relay coil 85 closes switch contacts 46 to connect the antenna switch relay coil I9 to the `110 volt line 3| through lines 45 and 41, now closed switch contacts 93 of the overload relay holding coil 9|, and line 83. The relay coil I9 is permanently connected to the volt line 25 through lines 44 and 43a. Thus under local control operation, when overload conditions exist to operate the overload relay 99 and energize its holding coil 9|, the power amplifier II is deenergized and the antenna I3 is switched from the output of the power amplifier II to the output of the exciter I8. It should be noted that relay coil 3| is also deenergized upon the opening of the normally closed switch contacts 'I6 to open switch contacts 61 and close switch contacts 61A and disconnect the output of the exciter from the input of the power amplifier and to the antenna line I5.

As soon as the local push-to-talk switch 58 is released to deenergize the plate supplies of both the exciter and power amplifier as previously described, the overload relay holding coil 9| is deenergized due to the opening of the grounding switch contacts 'I1 and the normally closed switch contacts 18 and 82 are again closed so that a subsequent closing of the local push-totallr switch 58 causing energization of relay coil 1| willgenergize relay coils I8 and 85 to energize the exciter and power amplifier and switch the antenna i3 from the exciter to the Apower amplifier. It will therefore be understood that relay and circuit means have been described to automatically switch ythe antenna from output of the power amplier to the output of the exciter while at the same time deenergizing the power amplifier in the event of overload conditions and to automatically reenergize the power amplifier and switch its output again to the antenna following a release and subsequent closing of `the local push-to-talk button when the transmitter system is operating under local control.

The details and operation of the transmitter control sys-tem under remote control conditions will now be described. It will be assumed that the transmitter warm-up procedure has been followed and only those elements of the system that are essential to an understanding of the remote control operation will be again described. The manual selec-tor switch $8 is set in the mid or normal position, as shown, for the remote control operation. A remote source of direct current |80 is yprovided to energize relays I8I and |82 upon closing of the remote push-to-talk microphone switch |03. A resistor condenser network comprising the resistor |04 and-con denser |05 is connected to provide a time delay so that the relay coil is energized an appreciable time before relay coil |02. IThe energization of relay coil |0| closes switch contacts |00 and |01, the closing of switch contacts |06 effecting the same result in energizing the exciter l0 as 4previously described when the local .pushto-talk switch 50 is closed. After the abovementioned time interval suiiicient to assure energization of the exciter, the relay coil |02 becomes energized to close switch contacts |08 and ground line 'l5 to cause a subsequent energization of the power amplifier as has been described. before when line I was grounded through switch contacts i3 and TI which were closed by the previously described local control operation of the selector switch 60 of the transmitter. rThe automatic overload responsive switching system operates for remote control in the same manner as previously described for the local control operation except that line 'I5 is now grounded through the closed switch contacts |01 and |08. Similarly, to restore the power ampliiier to full operation after an overload switch over it is only necessary to release and subsequently depress the remote .push button |03.

It will be noted that the relay coil |02 is also provided with grounding switch contacts |05 which are closed when the coil is energized. Thus the energization of the exciter |0 is controlled by either the closing of switch contacts |06 or |09. When the 4push button |03 is released, relay coil I0| is immediately deenergized to open switch contacts |06, |01 and deenergize only the power amplifier of the transmitter. After a time sufficient to discharge condenser |05, the relay coil |02 is deenergized -to open switch contacts |09 and since switch contacts 06 have been previously opened the exciter |0 is then deenergized. It will therefore be understood that the resistor |04 and condenser |05 together with the arrangemerit and connection of switch contacts |06-|09 as described above provides for the automatic energization of the exciter before the power amplifier and a subsequent deenergization of the power amplier before the exciter under remote control conditions of operation.

Now referring to Fig. 2 of the drawings, a modiiied form of the invention providing for automatic switching `of the antenna to the exciter circuit whenever the iinal amplier is deenergized and further providing for automatic switching of the antenna to a receiver whenever both the final ampliiier and exciter are deenergized will be described. The circuit of Fig. 2 is very similar to the .previously described circuit of Fig. 1 and all circuit elements in Fig. 2 which function the same as corresponding elements in Fig. 1 will be given the same reference numeral and will not be yfurther described in detail. It will be noted that in Fig. 2, the relay coil 4| and switch contacts 49 of Fig. 1 have been omitted, and a direct connection I0 from switch contacts 4B to line 3| has been inserted. This change is effective to cause relay coil i9 to become energized every time that relay coils |8 and 85 are deenergized to deenergize the final ampliiier either under overload conditions when relay coil has been deenergized and the normally closed switch contacts 48 have been closed and the normally open switch contacts 30 have been opened or when only the exciter is energized or purposes of tuning. Since the relay coil 1| is a-lso deenergized whenever the local or remote control push buttons 50 or |03 are released, it is apparent that the antenna I3 will be switched to line I5 whenever the rplate power to the final ampliiier is disconnected.

As shown in Fig. 2 of the drawings, in order that the antenna connection through switch contacts |'I and line |5 may be made with the output of the exciter |0 whenever overload conditions occur in the final amplifier, the switch contacts |20 and |20A shown in their normal position are provided to be operated by the energization of relay coil |2|, which is connected in parallel with the overload holding coil relay Sl. It will be remebered that relay coil 6| is deenergized whenever the overload relay holding coil 0| is energized, thus opening switch contacts 61 and closing switch contacts 61A to disconnect the output of the exciter from the input of the iinal amplifier yand to the antenna in line |5.

The receiver shown by block outline at |29 in Fig. 2 may be controlled by the same control cir- .cuit as the transmitter, and may be connected to the antenna I3 whenever the local or remote push-to-talk buttons are released. As previously described, relay coils 5| and 52 are energized whenever either push button 50 or |03 is depressed. The antenna of the receiver |20 is connected by line |33 with line I5 through switch contacts |20, line 68, and switch contacts 61A to thus connect the antenna through antenna switch contacts Il which are always closed when the plate power of the final amplifier is deenergized as previously described above. An additional pair of normally closed switch contacts |32 are provided to be operated by relay 5| to supply plate power to the receiver whenever relay 5| has been deenergized in the manner to deenergize the plate circuits of the exciter. In such manner automatic switching of the antenna to the receiver is accomplished whenever the remote or local push-to-talk buttons are released without interfering with the relay circuits providing the switching operations of the antenna from the final ampliiier to the exciter during overload conditions in any transmitting period.

In the foregoing, a transmitter control system has been described which is operative for either local or remote control and which provides an automatic switchover from high power to low power in the event of overload of the high power circuits and which further provides a sequential energization and deenergization of the various exciter and power amplifier units of the transmitter when operated under remote or local control conditions. When operated under local control conditions the desired sequential energization is obtained by the movement of the manual selector switch 60 to the local control position `and the closure of the control switch 40 to cause the power amplifier to be energized before the exciter can be energized by operation of the local push-to-talk switch 50. The local push-to-talk switch 50 is not effective until switches 40 and 00 have been operated as described. Control switch 40 may be independently opened to allow the exciter to be operated without operating the main power amplier in case of repairs to the main power amplifier.

In the event of failure of the blower motor 24 or fan 27, the air switch 28 is opened to disable the filament and high voltage circuits of the power amplifier of the transmitter at the same time switching the antenna to the exciter or receiver. Original conditions can only be restored to operation by a subsequent closing of the air switch. However, a switchover of the antenna from power amplier to exciter will be switched back automatically upon release and subsequent depression of either the local or remote push-totalk button switches and the transmitter will 4again operate at full power until another overof the invention as defined in the appended claims.

I claim:

l. A radio frequency communication system including in combination, a receiver, a transmitter having high and low power stages, an antenna, a push-to-talk switch, rst normally open switch means adapted when closed to energize said low power stage, second normally open switch means adapted when closed to energize said high power stage, a iirst antenna switch for switching said antenna between the input circuit -of said receiver and the output circuit of said high power transmitter stage, means responsive to the closing of said push-to-talk switch for closing said rst and second switch means, means responsive to operation of said push-to-talk switch for operating said antenna switch to connect said antenna to the input circuit of said receiver when said push-to-tall; switch is released and to connect said antenna to the output circuit of said high power stage when said push-totali: switch is closedy a second antenna switch to transfer the antenna from the output of said high power stage to the output of said low power stage, said high power stage including means connected to said second switch means and responsive to an overload condition of said high power stage for opening said second switch means, said overload responsive means also operating said second antenna switch to transfer the antenna to the output of said low power stage.

2. A radio frequency communication system including in combination, a receiver, a transmitter having separately energized high and low power stages, an antenna, a push-to-talk switch, rst normally open switch means adapted when closed to energize said low power stage, second normally open switch means adapted when closed to energize said high power stage, a first antenna switch for selectively connecting said antenna to the input circuit of said receiver and to the output circuit of said high power stage, a second antenna switch for switching said antenna between the output circuit of said high power stage and the output circuit of said lowpower stage, means responsive to the closing of said push-to-talk switch for closing said rst and second switch means, means responsive to operation of said push-to-tallr switch for operating 'said iirst antenna switch to connect said antenna to the input circuit of said receiver when said push-to-talk switch is released and to connect said antenna to the output circuit of said high power stage when said push-to-talk switch is closed, and means responsive to an overload condition in said high power stage to operate said second antenna switch and switch said antenna from the output circuit of the high power stage to the output circuit of the low power stage.

3. In a control circuit for a radio frequency communication system including in combination, a receiver, a transmitter having separately energized high and low power output stages, an antenna, a push-to-talk switch, iirst normally open switch means adapted when closed to energize said low power stage, second normally open switch means adapted when closed to energize said high power stage,a first antenna switch for selectively connecting said antenna to the input circuit of said receiver and to the output circuit of said low power stage, a second antenna switch normally closed to connect said antenna to the output circuit of said low power stage and operable to switch the antenna to the output circuit of said high power stage, means responsive to the closing of said push-to-talk switch for closing said rst and second switch means, means to delay the closing of said second switch means for a period of time following the closing of said iirst switch means, means to operate said rst antenna switch to connect said antenna to the input circuit of said receiver when said push-totalk switch is released, means for connecting the output circuit of said low low power stage to the input circuit of said high power stage and for connecting said second antenna switch to connect said antenna to the output circuit of said high power stage when said push-to-talk switch is closed, and means responsive to an overload condition in said high power stage to operate both said second antenna switch and said connecting means to thereby switch said antenna from the output circuit of the high power stage to the output circuit of the low power stage and to disconnect the output circuit of the low power stage from the input circuit of the high power stage.

4. In a control circuit for a radio communication system including in combination, a transmitter having separately energized high and low power stages, an antenna, a remote push-to-talk switch adapted to be connected to energize when closed said high and low power stages, a local pushto-talk switch adapted to be connected to energize when closed said high and low power stages, a manual selector switch having a plurality of selector positions, a rst selector position connecting said remote push-to-talk switch to control the energization of said high and low power stages, a second selector position connecting said local push-to-talk switch to control the energization of said high and lower power stages, and a third selector position modifying the connections of said local push-to-talk switch to control the energization only of said low power stage for purposes of tuning the low power stage, rst antenna switch means operative in response to an overload condition of said high power stage for switching said antenna from the output circuit of said high power stage to the output circuit of said low power stage, and switching means responsive to the operation of said overload condition responsive means for disconnecting the output circuit of said low power stage from the input circuit of said high power stage, said switching means also being responsive to the operation of said selector switch in the third selector position to connect the output circuit of said low power stage to the input circuit of l1 said high power stage for purposes of loading and tuning the low power stage.

5. In a radio communication system the combination including a transmitter having separately energized low and high power stages, a receiver, an antenna, a push-to-talk switch, first normally open switch means adapted when closed to energize said low power stage, second normally open switch means adapted when closed to energize said high power stage, antenna switching means for selectively connecting said antenna to said receiver and to said high and low power stages, circuit means responsive to operation of said push-to-talk switch for closing said first switch means, for closing said second switch means after a period of delay, and for operating said antenna switching means to connect said antenna to said high power stage, and means responsive to an overload condition of said high power stage for operating said antenna switching means to disconnect said antenna from said high power stage and to connect the same to said low power stage.

6. In a radio communication system the combination including a transmitter having separately energized low and high power stages, a receiver, an antenna, a push-to-talk switch, rst normally open switch means adapted when closed to energize said low power stage, second normally open switch means adapted when closed to energize said high power stage, antenna switching means for selectively connecting said antenna to said receiver and to said high and low power stages, circuit means responsive to operation of said push-to-talk switch for closing said first switch means, for closing said second switch means after a period of delay, and for operating said antenna switching means to ccnnect said antenna to said high power stage, said circuit means acting in response to release of said push-to-talk switch to operate said antenna switching means to connect said antenna to said receiver, to open said second switch means and to open said first switch means after a period of delay, and means responsive to an overload condition of said high power stage for operating said antenna switching means to disconnect said antenna from said high power stage and to connect the same to said low power stage, said last named means also operating to deenergize said high power stage in response to an overload condition.

7. In a radio communication system the combination including, a transmitter having separately energized high and low power output stages, an antenna, rst antenna relay means adapted when actuated to connect the output circuit of said high power stage to said antenna, second antenna relay means adapted when actuated to connect the output circuit of said low power stage to said antenna, third relay means adapted when actuated to energize said high power output stage, and overload relay means actuated in response to an overload condition of said high power stage, said overload relay means including contacts normally closed for holding said rst and third relay means actuated and opened by actuation of said overload relay means to release said first and third relay means to disconnect said high power stage from said antenna and deenergize said high power stage, said third relay means including contacts held open when said third relay means is actuated and closed in response to release of said third relay means for actuating said second 12 relay means to connect said low power stage to said antenna.

8. In a radio communication system the combination including, a transmitter having separately energized high and low power stages, an antenna, antenna relay means including a rst portion adapted when actuated to connect the output circuit of said high power stage to said antenna, and a second portion adapted when actuated to connect the output circuit of said low power stage to said antenna, power relay means adapted when actuated to energize said high power output stage, coupling relay means adapted when actuated to connect the output circuit of said low power stage to the input circuit of said high power stage, and overload relay means actuated in response to an overload condition of said high power stage, said overload relay means including contacts normally closed for holding said first portion of said antenna relay means and said power relay means actuated, said contacts being opened by actuation of said overload relay means to release said first portion of said antenna relay means and to release said power relay means, said power relay means including contacts held open when said power relay means is actuated and closed in response to release of said power relay means for actuating said second portion of said antenna relay means to connect said low power stage to said antenna, said overload relay means including contacts normally closed for holding said coupling relay means actuated and opened by actuation of said overload relay means to disconnect said high power stage from said low power stage.

9. In a radio communication system the combination including a transmitter having separately energized low and high power stages, a receiver, an antenna, a push-to-talk switch, first normally open switch means adapted when closed to energize said low power stage, second normally open switch means adapted when closed to energize said high power stage, antenna switching means normally connecting said antenna to said receiver and selectively operable for connecting said antenna to said high and low power stages, circuit means operating in response to operation of said push-to-talk switch for closing said rst and second switch means for energizing said low power stage and said high power stage, said circuit means also operating in response to operation of said push-totalk switch to operate said antenna switching means to connect said antenna to said high power stage, and means responsive to an overload condition of said high power stage for automatically operating said antenna switching means to disconnect said antenna from said high power stage and to connect the same to said low power stage, said overload responsive means also opening said second switch means for deenergizing said high power stage.

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